Smells, learning and memory
After seeing this article’s title, you might have thought: “That sounds rather boring. I mean, what is so interesting about the nose?” Perhaps the “memory” part aroused your curiosity, though. If that’s so, you might find the following reading worth having a look at, as you could discover some surprising things about the “nose”.
I’d like to begin by emphasising something important: we don’t actually smell with our noses; it’s the brain that identifies different odours through the central olfactory pathways, but we’ll get to that soon. What does happen in our nasal cavity is the activation of the olfactory receptors (a type of neurone) of the primary olfactory system, by chemical stimuli called odourants. The binding of odourants to the olfactory receptors’ cilia triggers the transduction process, which involves G-protein stimulation, formation of the cyclic AMP (cAMP) and membrane depolarisation, by the opening of ion channels (calcium, sodium and chloride). This complex signalling cascade results in a receptor potential which is then coded as an action potential (provided the receptor potential reaches a certain threshold) and then transmitted further along the receptor’s axons (remember, they are actually neurones!) The axons form the olfactory nerve, but they also group in small clusters and converge onto the two olfactory bulbs, in spherical structures known as glomeruli. Here, the axons synapse upon second-order neurones which form the olfactory tracts and finally project to the olfactory cortex (involved in the perception of smell) and to some structures in the temporal lobes, the medial dorsal nucleus (in the thalamus) and the orbitofrontal cortex. The last two are thought to play an important role in the conscious perception of smells. A pretty intricate process, isn’t it? But it is a lot more to olfaction than this!
Running parallel to the primary olfactory system is the accessory olfactory system. This has been shown to detect our favourite smelling chemicals, the pheromones. As I am sure most of you are aware of, pheromones are involved in reproductive behaviours, identifying individuals, aggression and submission recognition. Not only the type of chemical stimuli, but also the structures in the accessory olfactory system are different: the vomeronasal organ in the nasal cavity, the accessory olfactory bulb and last but not least the hypothalamus and amygdala (and hippocampus) as the final axonal targets. The amygdala and hippocampus are known for their implications in emotions and long-term memories (check out article about memory). Thus, olfaction also plays an important role in the integration of different odours in emotion processes, as well as explicit memory and associative meanings to odours.
Interestingly, each receptor cell is defined by only one receptor protein, which is encoded by a single receptor gene. These genes form the largest family of mammalian genes: 1000 in rodents, 350 in humans. The receptor cells have unique structures and are divided into different types according to their sensitivity to odours: each receptor type is activated by a single odour; nevertheless, one odour can activate many receptor types and the combination as well as the frequency, rhythmicity and temporal pattern of receptor stimulations encode for odour information.
Studies in Drosophila have shown another very important function of the olfactory processes: the associative learning. Gustatory unconditional and odour conditional signals both converge on the antennal lobe and mushroom body of the Drosophila, establishing learning efficacy of appetitive and aversive memories in classical conditioning. The release of certain catecholaminergic neurotransmitters such as dopamine and octopamine (the insect analogue of noradrenaline) are involved in the aversive and appetitive behaviours, respectively. In an incredibly revealing study, Lee Chi-Yu and his colleagues developed this topic in much more detail. I strongly recommend you have a look at it here.
As you might have guessed, given the fact that olfaction has a wide range of implications, its impairments are present in different mental diseases. Olfaction deficits or absence (anosmia) have been identified in Alzheimer’s disease and dementia, whereas olfactory hallucinations and weird smells are one of the main symptoms of schizophrenia.
Hopefully, you didn’t find this article too long or confusing. Did you find out new information about olfaction? If you have any questions or comments, please feel free to upload your posts. I’m looking forward to them as always.
For further information:
Bear et al., “Neuroscience”, third edition, Lippincott Williams & Wilkins
Photo by Isuru Priyaranga